Non-aqueous electrolyte secondary battery

Abstract

The purpose of this application is to provide a non-aqueous electrolyte secondary battery capable of suppressing the degradation of fast-charging cycle characteristics. As one embodiment of this application, the non-aqueous electrolyte secondary battery has a positive electrode, a negative electrode, and a non-aqueous electrolyte. The negative electrode has a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector. The negative electrode active material layer contains graphite particles A and graphite particles B as negative electrode active materials. The internal porosity of the graphite particles A is less than 5%, and the internal porosity of the graphite particles B is 8% to 20%. Compared to the region on the negative electrode current collector side when the negative electrode active material layer is bisected in the thickness direction, the graphite particles A are more abundant in the region on the outer surface side. The non-aqueous electrolyte has a non-aqueous solvent comprising a carboxylic acid ester represented by the following general formula (I), wherein the carboxylic acid ester contains 5% to 50% by volume relative to the total non-aqueous solvent (where R1 and R2 are independent and are hydrocarbon groups with 2 or fewer carbon atoms).

Description

Technical Field

[0001] This application relates to non-aqueous electrolyte secondary batteries. Background Technology

[0002] Non-aqueous electrolyte secondary batteries that use carbon materials as the negative electrode active material are widely used as high-energy-density secondary batteries.

[0003] For example, Patent Document 1 discloses a non-aqueous electrolyte secondary battery using carbon material with a densified carbon internal porosity of less than 5%.

[0004] For example, Patent Document 2 discloses a non-aqueous electrolyte secondary battery using carbon materials comprising carbon material A with an internal porosity of 1% or more and less than 23% and carbon material B with an internal porosity of 23% or more and less than 40%.

[0005] Existing technical documents

[0006] Patent documents

[0007] Patent Document 1: Japanese Patent Application Publication No. 9-320600

[0008] Patent Document 2: Japanese Patent Application Publication No. 2014-67638 Summary of the Invention

[0009] The problem that the invention aims to solve

[0010] However, in existing non-aqueous electrolyte secondary batteries, if fast charging cycles at high rates are performed to shorten charging time, there is a problem of reduced battery capacity.

[0011] Therefore, the purpose of this application is to provide a non-aqueous electrolyte secondary battery capable of suppressing the degradation of fast charging cycle characteristics.

[0012] means for solving problems

[0013] As one aspect of this application, a non-aqueous electrolyte secondary battery has a positive electrode, a negative electrode, and a non-aqueous electrolyte. The negative electrode has a negative current collector and a negative active material layer disposed on the negative current collector. The negative active material layer contains graphite particles A and graphite particles B as negative active materials. The internal porosity of the graphite particles A is less than 5%, and the internal porosity of the graphite particles B is 8% to 20%. Compared with the region on the negative current collector side when the negative active material layer is divided into two equal parts in the thickness direction, the graphite particles A are more contained in the region on the outer surface side. The non-aqueous electrolyte has a non-aqueous solvent containing a carboxylic acid ester represented by the following general formula (I). The carboxylic acid ester contains 5% to 50% by volume relative to the total non-aqueous solvent.

[0014] [Chemical Formula 1]

[0015]

[0016] (In the formula, R1 and R2 are independent and are hydrocarbon groups with 2 or fewer carbon atoms.)

[0017] Invention Effects

[0018] According to one aspect of this application, the degradation of fast charging cycle characteristics can be suppressed. Attached Figure Description

[0019] Figure 1 This is a cross-sectional view of a non-aqueous electrolyte secondary battery as an example of an implementation method.

[0020] Figure 2 This is a cross-sectional view of the negative electrode as an example of an implementation method.

[0021] Figure 3 This is a cross-sectional view of graphite particles within the negative electrode active material layer. Detailed Implementation

[0022] As one aspect of this application, a non-aqueous electrolyte secondary battery has a positive electrode, a negative electrode, and a non-aqueous electrolyte. The negative electrode has a negative current collector and a negative active material layer disposed on the negative current collector. The negative active material layer contains graphite particles A and graphite particles B as negative active materials. The internal porosity of the graphite particles A is less than 5%, and the internal porosity of the graphite particles B is 8% to 20%. Compared with the region on the negative current collector side when the negative active material layer is divided into two equal parts in the thickness direction, the graphite particles A are more contained in the region on the outer surface side. The non-aqueous electrolyte has a non-aqueous solvent containing a carboxylic acid ester represented by the following general formula (I). The carboxylic acid ester contains 5% to 50% by volume relative to the total non-aqueous solvent.

[0023] [Chemical Formula 2]

[0024]

[0025] (In the formula, R1 and R2 are independent and are hydrocarbon groups with 2 or fewer carbon atoms.)

[0026] Graphite particles A with an internal porosity of less than 5% can suppress the decomposition reaction of the non-aqueous electrolyte compared to graphite particles with an internal porosity of more than 5%. Furthermore, since the decomposition reaction of the non-aqueous electrolyte mainly occurs on the negative electrode surface, it can be assumed that, as in this application, by including more graphite particles A with an internal porosity of less than 5% in the outer half of the negative electrode active material layer compared to the negative electrode current collector side half, the decomposition reaction of the non-aqueous electrolyte is suppressed even under harsh conditions such as fast charging. On the other hand, graphite particles A with an internal porosity of less than 5% are less prone to breakage during electrode formation, thus exhibiting lower adhesion to the negative electrode current collector compared to graphite particles with an internal porosity of more than 5%. However, in this application, the region on the negative electrode current collector side of the negative electrode active material layer contains fewer graphite particles A with an internal porosity of less than 5%. Therefore, it is considered that the adhesion between the negative electrode current collector and the graphite particles is easily ensured, and the graphite particles are not easily peeled off from the negative electrode current collector even under harsh conditions such as fast charging. Furthermore, the non-aqueous electrolyte containing a specified amount of the aforementioned carboxylic acid ester has a lower viscosity than the non-aqueous electrolyte without the aforementioned carboxylic acid ester, resulting in improved permeability to the positive / negative electrode active material layers. Therefore, it is considered that even under harsh conditions such as fast charging, efficient battery reactions can occur. Based on the above, it is considered that the reduction in fast charging cycle characteristics is suppressed in the non-aqueous electrolyte secondary battery according to this application.

[0027] Hereinafter, an example of an embodiment will be described in detail with reference to the accompanying drawings. It should be noted that the non-aqueous electrolyte secondary battery of this application is not limited to the embodiment described below. Furthermore, the accompanying drawings referred to in the description of the embodiment are schematic illustrations.

[0028] Figure 1 This is a cross-sectional view of a non-aqueous electrolyte secondary battery as an example of an implementation method. Figure 1 The non-aqueous electrolyte secondary battery 10 shown includes: a wound electrode body 14 formed by winding a positive electrode 11 and a negative electrode 12 with a spacer 13 between them; a non-aqueous electrolyte; insulating plates 18 and 19 respectively disposed above and below the electrode body 14; and a battery casing 15 housing the above components. The battery casing 15 consists of a bottomed cylindrical casing body 16 and a sealing body 17 that closes the opening of the casing body 16. It should be noted that other types of electrode bodies, such as a stacked electrode body in which the positive and negative electrodes are alternately stacked with a spacer between them, can be used instead of the wound electrode body 14. In addition, examples of battery casing 15 include cylindrical, square, coin-shaped, button-shaped, and other metal outer cans, as well as bag-type outer cans formed by laminating resin sheets and metal sheets.

[0029] The casing body 16 is, for example, a bottomed cylindrical metal outer can. A sealing gasket 28 is provided between the casing body 16 and the sealing body 17 to ensure the airtightness of the battery interior. The casing body 16 has, for example, a portion of its side surface that bulges inward and supports the sealing body 17, a bulge 22. The bulge 22 is preferably formed in a ring shape along the circumference of the casing body 16, and supports the sealing body 17 on its upper surface.

[0030] The sealing body 17 has a structure in which a filter element 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cover 27 are stacked sequentially from the electrode body 14 side. Each component constituting the sealing body 17 has, for example, a circular or annular shape, and all components except the insulating member 25 are electrically connected to each other. The lower valve body 24 and the upper valve body 26 are connected to each other at their respective central portions, and the insulating member 25 is sandwiched between their respective peripheral portions. If the internal pressure of the non-aqueous electrolyte secondary battery 10 rises due to heat generated by internal short circuits, for example, the lower valve body 24 deforms and breaks by pushing the upper valve body 26 upwards towards the cover 27, thus cutting off the current path between the lower valve body 24 and the upper valve body 26. If the internal pressure rises further, the upper valve body 26 breaks, and gas is discharged from the opening of the cover 27.

[0031] exist Figure 1 In the non-aqueous electrolyte secondary battery 10 shown, the positive electrode lead 20, installed on the positive electrode 11, extends through the through hole of the insulating plate 18 towards the sealing body 17, and the negative electrode lead 21, installed on the negative electrode 12, extends through the outer side of the insulating plate 19 towards the bottom side of the housing body 16. The positive electrode lead 20 is connected to the bottom plate of the sealing body 17, i.e., the lower surface of the filter element 23, by welding or the like, and the top plate of the sealing body 17, i.e., the cover 27, which is electrically connected to the filter element 23, becomes the positive terminal. The negative electrode lead 21 is connected to the bottom inner surface of the housing body 16 by welding or the like, and the housing body 16 becomes the negative terminal.

[0032] The following is a detailed description of each component of the non-aqueous electrolyte secondary battery 10.

[0033] [negative electrode]

[0034] Figure 2 This is a cross-sectional view of the negative electrode as an example of an implementation. The negative electrode 12 has a negative electrode current collector 40 and a negative electrode active material layer 42 disposed on the negative electrode current collector 40.

[0035] The negative current collector 40 may be made of a foil of a metal such as copper that is stable in the potential range of the negative electrode, or a film of the metal disposed on the surface.

[0036] The negative electrode active material layer 42 contains graphite particles as the negative electrode active material. Furthermore, the negative electrode active material layer 42 preferably contains a binder or the like. The negative electrode 12 can be manufactured, for example, as follows: a negative electrode slurry containing the negative electrode active material, binder, etc., is prepared; the negative electrode slurry is coated onto the negative electrode current collector 40 and dried to form the negative electrode active material layer 42; and the negative electrode active material layer 42 is calendered. It should be noted that the details of the manufacturing method of the negative electrode active material layer 42 will be described later.

[0037] Figure 3 This is a cross-sectional view of graphite particles within the negative electrode active material layer. For example... Figure 3 As shown, in a cross-sectional view of the graphite particle 30, the graphite particle 30 has a closed void 34 (hereinafter referred to as internal void 34) that is not connected from the inside of the particle to the surface of the particle, and a void 36 (hereinafter referred to as external void 36) that is connected from the inside of the particle to the surface of the particle.

[0038] In this embodiment, the graphite particles 30 include graphite particles A with an internal porosity of 5% or less and graphite particles B with an internal porosity of 8% to 20%. Considering factors such as suppressing the reduction in fast charging cycle characteristics, the internal porosity of graphite particle A is preferably 1% to 5%, and more preferably 3% to 5%. Considering factors such as suppressing the reduction in fast charging cycle characteristics, the internal porosity of graphite particle B is preferably 8% to 20%, and more preferably 10% to 18%, and more preferably 12% to 16%. Here, the internal porosity of the graphite particles refers to a two-dimensional value calculated as the ratio of the area of ​​the internal voids 34 of the graphite particles to the cross-sectional area of ​​the graphite particles. Furthermore, the internal porosity of the graphite particles is calculated through the following steps.

[0039] <Methods for determining internal porosity>

[0040] (1) Exposing the cross-section of the negative electrode active material layer. For example, a method to expose the cross-section is to cut a portion of the negative electrode and process it with an ion milling device (e.g., Hitachi High-Tech Co., Ltd., IM4000PLUS) to expose the cross-section of the negative electrode active material layer.

[0041] (2) Using a scanning electron microscope, take a cross-sectional image of the exposed negative electrode active material layer. The magnification of the reflected electron image is 3,000 to 5,000 times.

[0042] (3) The cross-sectional image obtained above is read into the computer and binarized using image analysis software (e.g., ImageJ, made by the National Institutes of Health, USA) to obtain a binarized image in which the particle cross-sections in the cross-sectional image are converted into black and the gaps in the particle cross-sections are converted into white.

[0043] (4) Select graphite particles A and B with a diameter of 5μm to 50μm from the binarized image, and calculate the area of ​​the cross-section of the graphite particle and the area of ​​the internal voids existing in the cross-section of the graphite particle. Here, the area of ​​the cross-section of the graphite particle refers to the area of ​​the region surrounded by the outer periphery of the graphite particle, that is, the total area of ​​the cross-section of the graphite particle. In addition, for voids with a width of less than 3μm in the cross-section of the graphite particle, it is sometimes difficult to distinguish whether they are internal voids or external voids in image analysis. Therefore, voids with a width of less than 3μm can be regarded as internal voids. Then, calculate the internal porosity of the graphite particle (area of ​​internal voids in the cross-section of the graphite particle × 100 / area of ​​the cross-section of the graphite particle) based on the calculated area of ​​the cross-section of the graphite particle and the area of ​​the internal voids in the cross-section of the graphite particle. The internal porosity of graphite particles A and B is the average value of 10 of each of the graphite particles A and B.

[0044] Graphite particles A and B are manufactured, for example, as described below.

[0045] <Graphite particles A with an internal porosity of less than 5%>

[0046] For example, coke (precursor), which will become the main raw material, is pulverized to a specified size, and while it is agglomerated with a binder, it is calcined at a temperature of 2600°C or higher to graphitize it. After sieving, graphite particles A of the desired size are obtained. Here, the internal porosity can be adjusted to 5% or less based on the particle size of the pulverized precursor and the particle size of the agglomerated precursor. For example, the average particle size (median particle size D50 in volumetric terms) of the pulverized precursor is preferably in the range of 12 μm to 20 μm. Furthermore, when reducing the internal porosity to within the range of 5% or less, it is preferable to increase the particle size of the pulverized precursor.

[0047] <Graphite particles B with an internal porosity of 8%–20%>

[0048] For example, coke (precursor), which will become the main raw material, is crushed to a specified size, agglomerated with a binder, and then pressed into a block shape. This block is then fired at a temperature above 2600°C to graphitize it. By crushing and sieving the graphitized block, graphite particles B of the desired size are obtained. Here, the internal porosity can be adjusted to 8%–20% depending on the amount of volatile components added to the block. If a portion of the binder added to the coke (precursor) volatilizes during firing, the binder can be used as the volatile component. Asphalt can be cited as an example of such a binder.

[0049] The graphite particles A and B used in this embodiment are natural graphite, artificial graphite, etc., without particular restrictions. Considering the ease of adjusting the internal porosity, artificial graphite is preferred. The interplanar spacing (d) of the (002) plane of the graphite particles A and B used in this embodiment, based on X-ray wide-angle diffraction, is... 002 For example, the preferred size is 0.3354 nm or more, more preferably 0.3357 nm or more, and more preferably less than 0.340 nm, and more preferably less than 0.338 nm. Furthermore, the crystallite size (Lc(002)) of the graphite particles A and B used in this embodiment, determined by X-ray diffraction, is preferably 5 nm or more, more preferably 10 nm or more, and more preferably less than 300 nm, and more preferably less than 200 nm. The interplanar spacing (d) 002 When the crystallite size (Lc(002)) meets the above range, there is a tendency for the capacity of non-aqueous electrolyte secondary batteries to increase compared with the case where the above range is not met.

[0050] In this embodiment, with Figure 2 Compared to region 42a on the negative electrode current collector side when the negative electrode active material layer 42 is bisected in the thickness direction, graphite particles A are more abundantly contained in region 42b on the outer surface side. Therefore, during fast charging cycles, for example, the decomposition of the non-aqueous electrolyte can be suppressed, or the stripping of graphite particles from the negative electrode current collector 40 can be suppressed, thus preventing a decrease in fast charging cycle characteristics. It should be noted that bisecting the negative electrode active material layer 42 in the thickness direction means that when the stacking direction of the negative electrode current collector 40 and the negative electrode active material layer 42 is set to the thickness direction of the negative electrode active material layer 42, it is divided into two halves at the midpoint Z of the thickness of the negative electrode active material layer 42. Furthermore, in the thickness direction of the negative electrode active material layer 42, the negative electrode active material layer 42 located closer to the negative electrode current collector 40 is designated as the negative electrode current collector side half of the region 42a, and the negative electrode active material layer 42 located farther away from the negative electrode current collector 40 is designated as the outer surface side half of the region 42b.

[0051] In this embodiment, compared to the negative electrode current collector side region 42a, more graphite particles A are contained in the outer surface side region 42b. From the perspective of further suppressing the reduction in fast charging cycle characteristics, the mass ratio of graphite particles A to graphite particles B in the outer surface side region 42b is preferably 20:80 to 100:0, more preferably 50:50 to 100:0. Furthermore, from the perspective of further suppressing the reduction in fast charge / discharge cycle characteristics, the mass ratio of graphite particles A to graphite particles B in the negative electrode current collector side region 42a is preferably 10:90 to 0:100, more preferably 0:100.

[0052] An example of a method for fabricating the negative electrode active material layer 42 will be described. For example, a negative electrode active material containing graphite particles B (and graphite particles A if necessary), a binder, and a solvent such as water are mixed to prepare a negative electrode slurry for the negative electrode current collector side. Alternatively, a negative electrode active material containing a larger amount of graphite particles A (and graphite particles B if necessary) than the negative electrode slurry for the negative electrode current collector side, a binder, and a solvent such as water are mixed to prepare a negative electrode slurry for the outer surface side. Then, the negative electrode slurry for the negative electrode current collector side is coated on both sides of the negative electrode current collector side and dried. On the coating film formed by the negative electrode slurry for the negative electrode current collector side, the negative electrode slurry for the outer surface side is coated on both sides and dried, thereby forming the negative electrode active material layer 42. In the above method, after coating the negative electrode current collector side with the negative electrode slurry and drying it, the negative electrode slurry for the outer surface side is coated. However, it is also possible to coat the negative electrode current collector side with the negative electrode slurry after coating the negative electrode current collector side and before drying. Alternatively, the negative electrode current collector side and the negative electrode slurry for the outer surface side can be coated simultaneously.

[0053] In addition to the graphite particles A and B used in this embodiment, the negative electrode active material may also include other materials capable of reversibly absorbing and releasing lithium ions, such as Si-based materials. Examples of Si-based materials include Si, Si-containing alloys, and SiO₂. x Silicon oxides, etc. (x is 0.8 to 1.6). Si-based materials are negative electrode materials that can improve battery capacity compared to graphite particles, but on the other hand, they have large volume expansion with charging and discharging, which is detrimental to fast charging cycle characteristics. However, in a negative electrode active material layer having a negative electrode active material containing graphite particles A and B and a Si-based material, when the amount of graphite particles A is greater in the outer surface half of the region than in the negative electrode current collector half of the region, compared to the case where graphite particles A are uniformly dispersed in the negative electrode active material layer, the reduction in fast charging cycle characteristics can be suppressed. Considering the improvement of battery capacity and the suppression of the reduction in fast charging cycle characteristics, the content of Si-based material is preferably 1% to 10% by mass relative to the mass of the negative electrode active material, more preferably 3% to 7% by mass.

[0054] Other materials capable of reversibly absorbing and releasing lithium ions include, in addition to, metals alloyed with lithium such as tin (Sn), and alloys and oxides containing metallic elements such as Sn. The negative electrode active material may contain the aforementioned other materials, and the content of these other materials is preferably 10% by mass or less relative to the mass of the negative electrode active material.

[0055] Examples of adhesives include: fluorinated resins, PAN, polyimide resins, acrylic resins, polyolefin resins, styrene-butadiene rubber (SBR), nitrile rubber (NBR), carboxymethyl cellulose (CMC) or its salts, polyacrylic acid (PAA) or its salts (PAA-Na, PAA-K, etc., and partially neutralized salts), polyvinyl alcohol (PVA), etc. They can be used alone or in combination of two or more.

[0056] [positive electrode]

[0057] The positive electrode 11 is, for example, composed of a positive current collector such as a metal foil and a layer of positive active material formed on the positive current collector. The positive current collector can be a foil of a metal that is stable within the potential range of the positive electrode, such as aluminum, or a film of the metal disposed on its surface. The positive active material layer may include, for example, a positive active material, a binder, a conductive agent, etc.

[0058] For example, the positive electrode 11 can be manufactured by coating a positive electrode slurry containing positive electrode active material, binder, conductive agent, etc. onto the positive electrode current collector and drying it to form a positive electrode active material layer, and then calendering the positive electrode active material layer.

[0059] Examples of lithium transition metal oxides containing transition metal elements such as Co, Mn, and Ni can be cited as positive electrode active materials. For example, Li0.05 is a lithium transition metal oxide. x CoO2, Li x NiO2, Li x MnO2, Li x Co y Ni 1-y O2, Li x Co y M 1-y O z Li x Ni 1- y M y O z Li x Mn2O4, Li x Mn 2-y M y O4, LiMPO4, Li2MPO4F (M; at least one of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, B, 0 < x ≤ 1.2, 0 < y ≤ 0.9, 2.0 ≤ z ≤ 2.3). These can be used individually or in combination. From the perspective of achieving high capacity in non-aqueous electrolyte secondary batteries, the positive electrode active material preferably contains Li. x NiO2, Li x Co yNi 1-y O2, Li x Ni 1-y M y O z (At least one of M, Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, B, 0 < x ≤ 1.2, 0 < y ≤ 0.9, 2.0 ≤ z ≤ 2.3) and other lithium nickel composite oxides.

[0060] Conductive agents include, for example, carbon black (CB), acetylene black (AB), Ketjen black, graphite, and other carbon-based particles. They can be used alone or in combination of two or more.

[0061] Examples of adhesives include fluorinated resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide resins, acrylic resins, and polyolefin resins. They can be used alone or in combination of two or more.

[0062] [Non-aqueous electrolyte]

[0063] The non-aqueous electrolyte contains a non-aqueous solvent comprising a carboxylic acid ester represented by the following general formula (I). Furthermore, the carboxylic acid ester represented by the following general formula (I) comprises 5% to 50% by volume relative to the total non-aqueous solvent.

[0064] [Chemical Formula 3]

[0065]

[0066] (In the formula, R1 and R2 are independent and are hydrocarbon groups with 2 or fewer carbon atoms.)

[0067] By using a non-aqueous electrolyte containing a specified amount of carboxylic acid ester represented by the above general formula (I), for example, the increased permeability to the positive / negative electrode active material layers due to the lower viscosity of the non-aqueous electrolyte enables efficient battery reactions even under harsh conditions such as fast charging, suppressing the deterioration of fast charging cycle characteristics. If the content of the carboxylic acid ester represented by the above general formula (I) exceeds 50% by volume relative to the total non-aqueous solvent, for example, the non-aqueous electrolyte is prone to decomposition, and the deterioration of fast charging cycle characteristics cannot be sufficiently suppressed. From the perspective of suppressing the deterioration of fast charging cycle characteristics and exhibiting excellent battery resistance at low temperatures, the content of the carboxylic acid ester represented by the above general formula (I) is preferably in the range of 30% to 50% by volume relative to the total non-aqueous solvent, more preferably in the range of 35% to 45% by volume.

[0068] Examples of hydrocarbon groups with two or fewer carbon atoms include alkyl groups with two or fewer carbon atoms, alkynyl groups with two or fewer carbon atoms, and alkenyl groups with two or fewer carbon atoms. Examples of alkyl groups with two or fewer carbon atoms include methyl and ethyl. Examples of alkynyl groups with two or fewer carbon atoms include ethynyl. Examples of alkenyl groups with two or fewer carbon atoms include vinyl. The hydrogen atoms of alkyl, alkenyl, and alkynyl groups can be replaced by different elements as needed. There are no particular limitations on the different elements; examples include fluorine, chlorine, and bromine.

[0069] Examples of carboxylic acid esters represented by the above general formula (I) include methyl acetate, ethyl acetate, methyl propionate, and ethyl propionate. Among these, methyl acetate is preferred from the viewpoint of further suppressing the degradation of fast charging cycle characteristics. These can be used alone or in combination of two or more.

[0070] Non-aqueous solvents may also include, for example, non-aqueous solvents other than carboxylic acid esters represented by the above general formula (I). Other non-aqueous solvents may include, for example, esters, ethers, nitriles such as acetonitrile, amides such as dimethylformamide, and mixtures of two or more of these solvents. Non-aqueous solvents may contain halogen-substituted products in which at least a portion of the hydrogen atoms of these solvents are replaced by halogen atoms such as fluorine.

[0071] Examples of the aforementioned esters include cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), and butyl carbonate; chain carbonates such as dimethyl carbonate (DMC), methyl ethyl carbonate (MEC), diethyl carbonate (DEC), methyl propyl carbonate, ethyl propyl carbonate, and methyl isopropyl carbonate; cyclic carboxylic acid esters such as γ-butyrolactone and γ-valerolactone; and chain carboxylic acid esters such as propyl acetate and propyl propionate. It should be noted that chain carboxylic acid esters do not include carboxylic acid esters represented by the above general formula (I).

[0072] Examples of the aforementioned ethers include 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, propylene oxide, 1,2-epoxybutane, 1,3-dioxane, 1,4-dioxane, 1,3,5-trioxane, furan, 2-methylfuran, 1,8-eucalyptol, crown ethers and other cyclic ethers, 1,2-dimethoxyethane, diethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, and ethylethylene. Chain ethers such as methyl ether, butyl vinyl ether, methyl phenyl ether, ethyl phenyl ether, butyl phenyl ether, pentyl phenyl ether, methoxytoluene, benzyl ethyl ether, diphenyl ether, dibenzyl ether, o-dimethoxybenzene, 1,2-diethoxyethane, 1,2-dibutoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol dibutyl ether, 1,1-dimethoxymethane, 1,1-diethoxyethane, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, etc.

[0073] As the halogen substitutes mentioned above, fluorocyclic carbonates such as fluoroethylene carbonate (FEC) and fluorochain carbonates are preferred.

[0074] The non-aqueous electrolyte contains an electrolyte salt. The electrolyte salt is preferably a lithium salt. Examples of lithium salts include LiBF4, LiClO4, LiPF6, LiAsF6, LiSbF6, LiAlCl4, LiSCN, LiCF3SO3, LiCF3CO2, Li(P(C2O4)F4), and LiPF6. 6-x (C n F 2n+1 ) x (1 < x < 6, n is 1 or 2), LiB 10 Cl 10 LiCl, LiBr, LiI, lithium chloroborane, lower aliphatic carboxylic acids, Li₂B₄O₇, Li(B(C₂O₄)F₂) and other borates, LiN(SO₂CF₃)₂, LiN(C l F 2l+1 SO2)(C m F 2m+1 Imidamine salts such as SO2 (where l and m are integers greater than or equal to 0) are used. Lithium salts can be used alone or in combination. Among these, LiPF6 is preferred from the viewpoints of ionic conductivity and electrochemical stability. The preferred concentration of the lithium salt is 0.8–1.8 mol per 1 L of non-aqueous solvent.

[0075] For example, from the perspective of further suppressing the reduction of fast charging cycle characteristics, the viscosity of the non-aqueous electrolyte is preferably 3.1 cP or less, more preferably 2.8 cP or less, and even more preferably 2.5 cP or less.

[0076] [Spacer]

[0077] The spacer 13 can be, for example, a porous sheet with ion permeability and insulation. Specific examples of porous sheets include microporous films, woven fabrics, and nonwoven fabrics. The preferred materials for the spacer are olefin resins such as polyethylene and polypropylene, and cellulose. The spacer 13 can be a laminate containing a cellulose fiber layer and a thermoplastic resin fiber layer such as an olefin resin. Alternatively, it can be a multilayer spacer containing a polyethylene layer and a polypropylene layer, or a spacer with an aromatic polyamide resin, ceramic, or other material coated on its surface.

[0078] Example

[0079] The present invention will be further illustrated below by way of examples, but the present invention is not limited to these examples.

[0080] <Example 1>

[0081] [The production of the positive electrode]

[0082] As the positive electrode active material, lithium nickel cobalt oxide (LiNi) containing aluminum is used. 0.88 Co 0.09 Al 0.03 O2). 100 parts by weight of the above-mentioned positive electrode active material, 1 part by weight of acetylene black, and 0.9 parts by weight of polyvinylidene fluoride were mixed in N-methyl-2-pyrrolidone (NMP) solvent to prepare a positive electrode slurry. This slurry was coated onto both sides of a 15 μm thick aluminum foil. After drying the coating, it was calendered using calendering rollers to create a positive electrode with positive electrode active material layers formed on both sides of the positive electrode current collector. The prepared positive electrode was cut into pieces with a width of 57.6 mm and a length of 679 mm for use.

[0083] [The Production of Graphite Particle A]

[0084] Coke was pulverized to an average particle size (median particle size D50) of 12 μm. Pitch was added as a binder to the pulverized coke, causing it to agglomerate to an average particle size (median particle size D50) of 17 μm. This agglomerate was then calcined at 2800 °C to graphitize, and sieved using a 250-mesh sieve to obtain graphite particles A with an average particle size (median particle size D50) of 23 μm.

[0085] [The Production of Graphite Particles B]

[0086] Coke was pulverized to an average particle size (median particle size D50) of 15 μm. Pitch was added as a binder to the pulverized coke to cause it to coagulate. Then, it was further subjected to isotropic pressure to produce a product with a particle size of 1.6 g / cm³. 3 ~1.9g / cm 3 A block-shaped molded body with a certain density was obtained. The block-shaped molded body was then graphitized by firing at a temperature of 2800°C. Next, the graphitized block-shaped molded body was crushed and sieved using a 250-mesh sieve to obtain graphite particles B with an average particle size (median particle size D50) of 23 μm.

[0087] [Making the negative electrode]

[0088] A mixture of 40 parts by mass of graphite particles A, 55 parts by mass of graphite particles B, and 5 parts by mass of SiO was used as negative electrode active material A, which was contained in the outer half of the negative electrode active material layer. A negative electrode slurry for the outer surface side was prepared by mixing negative electrode active material A with carboxymethyl cellulose (CMC) and styrene-butadiene copolymer rubber (SBR) in a mass ratio of 100:1:1. Conversely, a mixture of 95 parts by mass of graphite particles B and 5 parts by mass of SiO was used as negative electrode active material B, which was contained in the negative electrode current collector side of the negative electrode active material layer. A negative electrode slurry for the negative electrode current collector side was prepared by mixing negative electrode active material B with carboxymethyl cellulose (CMC) and styrene-butadiene copolymer rubber (SBR) in a mass ratio of 100:1:1.

[0089] A negative electrode slurry on the negative current collector side was coated onto both sides of an 8 μm thick copper foil. After drying the coating, a negative electrode slurry on the outer surface side was coated onto the coating and dried. The coating was then calendered using calendering rollers, thereby fabricating a negative electrode with negative electrode active material layers formed on both sides of the negative current collector. Specifically, the ratio of graphite particles A to graphite particles B in the outer half of the active material layer was 40:55 by mass, while the ratio in the negative electrode current collector side of the active material layer was 0:100 by mass. Furthermore, the internal porosity of graphite particles A and B in the fabricated negative electrode was measured, and the results were 3% and 15%, respectively. The fabricated negative electrode was cut into pieces with a width of 58.6 mm and a length of 662 mm for use.

[0090] [Preparation of non-aqueous electrolytes]

[0091] LiPF6 was dissolved in a non-aqueous solvent prepared by mixing ethylene carbonate (EC), methyl ethyl carbonate (MEC), dimethyl carbonate (DMC), and methyl acetate (MA) in a volume ratio of 16:4:70:10, at a concentration of 1.3 mol / L. This solution was used as a non-aqueous electrolyte. The viscosity of the non-aqueous electrolyte in Example 1 was 2.8 cP. The viscosity of the non-aqueous electrolyte was measured using a rotational viscometer. The following examples and comparative examples were also determined similarly.

[0092] [Fabrication of Non-Aqueous Electrolyte Secondary Batteries]

[0093] (1) After installing a positive lead in the positive current collector and a negative lead in the negative current collector, a polyethylene spacer is placed between the positive and negative electrodes to make a wound electrode body.

[0094] (2) Insulating plates are placed on the top and bottom of the electrode body respectively. The negative lead wire is welded to the main body of the shell, and the positive lead wire is welded to the sealing body. The electrode body is then stored inside the main body of the shell.

[0095] (3) After injecting non-aqueous electrolyte into the main body of the casing by depressurization, the opening end of the main body of the casing is sealed with a sealing body through the sealing gasket. It is used as a non-aqueous electrolyte secondary battery.

[0096] <Example 2>

[0097] LiPF6 was dissolved in a non-aqueous solvent prepared by mixing ethylene carbonate (EC), ethyl methyl carbonate (MEC), dimethyl carbonate (DMC), and methyl acetate (MA) in a volume ratio of 14:3:53:30, at a concentration of 1.3 mol / L. This solution was used as the non-aqueous electrolyte. The viscosity of this non-aqueous electrolyte was 2.4 cP. In Example 2, a non-aqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that the above-prepared non-aqueous electrolyte was used.

[0098] <Example 3>

[0099] LiPF6 was dissolved in a non-aqueous solvent prepared by mixing ethylene carbonate (EC), ethyl methyl carbonate (MEC), dimethyl carbonate (DMC), and ethyl propionate (PE) in a volume ratio of 16:4:70:10, at a concentration of 1.3 mol / L. This solution was used as the non-aqueous electrolyte. The viscosity of this non-aqueous electrolyte was 3.1 cP. In Example 3, a non-aqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that the above-prepared non-aqueous electrolyte was used.

[0100] <Comparative Example 1>

[0101] LiPF6 was dissolved in a non-aqueous solvent prepared by mixing ethylene carbonate (EC), ethyl methyl carbonate (MEC), and dimethyl carbonate (DMC) in a volume ratio of 20:5:75, at a concentration of 1.3 mol / L. This solution was used as the non-aqueous electrolyte. The viscosity of this non-aqueous electrolyte was 3.2 cP. In Comparative Example 1, the non-aqueous electrolyte prepared above was used, and a non-aqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that...

[0102] <Comparative Example 2>

[0103] A mixture of 95 parts by mass of graphite particles B and 5 parts by mass of SiO was used as the negative electrode active material C throughout the entire region of the negative electrode active material layer. A negative electrode slurry was prepared by mixing negative electrode active material C, carboxymethyl cellulose (CMC), and styrene-butadiene copolymer rubber (SBR) in a mass ratio of 100:1:1. This negative electrode slurry was coated on both sides of an 8 μm thick copper foil. After drying the coating, it was calendered using calendering rollers to create a negative electrode with negative electrode active material layers formed on both sides of the negative electrode current collector. Specifically, the mass ratio of graphite particles A to graphite particles B in the outer half of the negative electrode active material layer was 0:100, and the mass ratio of graphite particles A to graphite particles B in the negative electrode current collector side of the negative electrode active material layer was also 0:100. The prepared negative electrode was cut into pieces with a width of 58.6 mm and a length of 662 mm for use.

[0104] In Comparative Example 2, the negative electrode prepared above and the non-aqueous electrolyte prepared in Comparative Example 1 were used to prepare a non-aqueous electrolyte secondary battery in the same manner as in Example 1.

[0105] <Comparative Example 3>

[0106] Using the negative electrode prepared in Comparative Example 1, a non-aqueous electrolyte secondary battery was prepared in the same manner as in Example 1.

[0107] [Determination of capacity retention during fast charging cycles]

[0108] The non-aqueous electrolyte secondary batteries of each embodiment and comparative example were charged at a constant current of 1C (4600mA) to 4.2V at an ambient temperature of 25°C, and then charged at a constant voltage of 4.2V to 1 / 50C. Then, they were discharged at a constant current of 0.5C to 2.5V. This charge-discharge cycle was considered one cycle, and 100 cycles were performed. The capacity retention rate of the non-aqueous electrolyte secondary batteries of each embodiment and comparative example during the fast charging cycle was then calculated using the following formula.

[0109] Capacity maintenance rate = (Discharge capacity in the 100th cycle / Discharge capacity in the 1st cycle) × 100

[0110] [Determination of DC resistance under low temperature conditions]

[0111] The non-aqueous electrolyte secondary batteries of each embodiment and comparative example were charged to 3.54V at a constant current of 0.3C (1C = 4700mA) at an ambient temperature of 25°C, and then charged to 1 / 50C at a constant voltage. After a 2-hour rest at an ambient temperature of -20°C, they were charged to 3.54V at a constant current of 0.3C. They were then discharged at 0.3C, and the voltage before discharge and the voltage 10 seconds after the start of discharge were measured. The measured voltages were applied to the following formula to calculate the DC resistance (DCR) of each battery under low-temperature conditions.

[0112] DCR(Ω) = (Voltage after 10 seconds - Voltage before discharge) / Current value

[0113] Table 1 summarizes the capacity retention rate and DCR (Discharge Rate Retention) results of the non-aqueous electrolyte secondary batteries of each embodiment and comparative example during fast charging cycles. It should be noted that a higher capacity retention rate during fast charging cycles indicates that the degradation of fast charging cycle characteristics is more suppressed.

[0114] [Table 1]

[0115]

[0116] Comparing Comparative Examples 1 and 2 with the same electrolyte composition, Comparative Example 1, which used a negative electrode in which graphite particles A were more abundant in the outer surface half of the negative electrode compared to the region on the negative electrode current collector side when the negative electrode active material layer was bisected in the thickness direction, showed a higher capacity retention rate during fast charging cycles and suppressed the decrease in fast charging cycle characteristics compared to Comparative Example 2. Furthermore, comparing Examples 1-3 and Comparative Example 1, which used the same negative electrode, Examples 1-3, which used a non-aqueous electrolyte containing 10% by volume or more of methyl acetate and ethyl propionate, showed a higher capacity retention rate during fast charging cycles and suppressed the decrease in fast charging cycle characteristics compared to Comparative Example 1. Among Examples 1-3, Examples 1 and 2, which used methyl acetate, showed a higher capacity retention rate during fast charging cycles and suppressed the decrease in fast charging cycle characteristics compared to Example 3. In particular, Example 2, which used a non-aqueous electrolyte containing 30% by volume or more of methyl acetate, had the highest capacity retention rate during fast charging cycles. Furthermore, Example 2 had the lowest DCR value at low temperatures among the examples and comparative examples.

[0117] Explanation of reference numerals in the attached figures

[0118] 10 Non-aqueous electrolyte secondary battery, 11 Positive electrode, 12 Negative electrode, 13 Spacer, 14 Electrode body, 15 Battery casing, 16 Casing body, 17 Sealing body, 18, 19 Insulating plate, 20 Positive electrode lead, 21 Negative electrode lead, 22 Bulging part, 23 Filter, 24 Lower valve body, 25 Insulating component, 26 Upper valve body, 27 Cover, 28 Sealing gasket, 30 Graphite particles, 34 Internal void, 36 External void, 40 Negative electrode current collector, 42 Negative electrode active material layer, 42a Half of the negative electrode current collector side area, 42b Half of the outer surface side area.

Claims

1. A non-aqueous electrolyte secondary battery, comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein, The negative electrode has a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector. The negative electrode active material layer contains graphite particles A and graphite particles B as negative electrode active materials. The internal porosity of graphite particle A is less than 5%, and the internal porosity of graphite particle B is 8% to 20%. Compared to the region on the negative electrode current collector side when the negative electrode active material layer is divided into two equal parts in the thickness direction, the graphite particles A are more abundantly contained in the region on the outer surface side. The non-aqueous electrolyte has a non-aqueous solvent comprising a carboxylic acid ester represented by the following general formula (I). The carboxylic acid ester contains 5% to 50% by volume relative to the total amount of the non-aqueous solvent. In the formula, R1 and R2 are independent and are hydrocarbon groups with 2 or fewer carbon atoms.

2. The non-aqueous electrolyte secondary battery according to claim 1, wherein, The carboxylic acid ester includes methyl acetate.

3. The non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein, The viscosity of the non-aqueous electrolyte is below 3.1 cP.

4. The non-aqueous electrolyte secondary battery according to any one of claims 1 to 3, wherein, The negative electrode active material comprises Si-based materials.

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